A couple of art inaccuracies that I remember are Sulu in a blue uniform shirt on the cover of Shadow Lord
, and a large sword being prominently featured on the cover of Captain's Honor
despite no sword being featured in the novel at all. And for something more recent, how about RIker's uniform on the cover of Titan: Synthesis
And yes, there were many many instances of mixing TOS uniform styles with refit Enterprises on the covers, and vice versa. But those cover paintings were very cool, especially the ones by Boris Vallejo.